In the central interior of British Columbia (Southern Interior Forest Region), Douglas-fir (Pseudotsuga menziesii) tends to occur in even-aged stands in the Interior Cedar-Hemlock Moist, Cool, Horsefly biogeoclimatic variant (ICHmk3). Douglas-fir stands are important from both forest industry and wildlife habitat management perspectives. Mule deer require mature and older Douglas-fir stands as winter range. In these ecosystems, Douglas-fir stands are typically clearcut, thereby seriously compromising habitat value as winter range. This is a pilot study to examine the response of vegetation (percent cover) and Douglas-fir regeneration (density and growth) to a range of opening sizes, opening orientation (along and across contours), and site preparation treatment (yes or no), 5 years post-harvest. The openings (15 × 165 m [0.25 ha], 30 × 165 m [0.5 ha], 60 × 165 m [1.0 ha], 60 × 330 m and 140 × 140 m [2.0 ha]) are options for group selection, patch cut, or clearcut silvicultural systems.
Although most of the 19 tree, shrub, and grass species that mule deer could eat did not change in percent cover from pre-harvest to 5 years postharvest, the species that did change were most strongly affected by harvesting, not opening size. A major diet component, western redcedar (Thuja plicata), was reduced from 9.6 to 1.4% in the site-prepared openings, and from 9.4 to 3.9% in the openings not site-prepared, when comparing the pre-harvest to the 5th-year post-harvest assessment. However, in the 5 years since harvesting, this species has increased from 533 stems per ha to 783 stems per ha (47%) and should increase steadily in cover over time. Of note was a big increase in red raspberry (Rubus idaeus) (from 0.1% up to 14%) and a moderate increase in birch-leaved spirea (Spirea betufolia) (from 1% up to 6%); however, they generally occur in small amounts (<1%) in mule deer diets.
Viable conifer seed was produced in every year, although amounts varied among species and years. The bulk of the viable seed for species of commercial interest, from 1998 to 2003, was from Douglas-fir, which is representative of the residual stand composition. Seedling establishment has been successful for all the commercial conifer species. In particular, the density of Douglas-fir natural regeneration and planted stock combined increased from 1500 stems per ha in 1998 to 2675 stems per ha in 2002. Overall, survival of planted Douglas-fir was 92% across openings and site preparation treatments (disc trenching); however, growth (height and diameter) was substantially reduced in the narrowest openings (15 m). Site preparation (specifically disc trenching) did not affect the planted stock response. Growth rates were similar on stock planted in openings that ran across contours and along contours. Seedlings planted within 2-5 m of a forest edge had lower survival, poorer condition, and lower growth rates than those planted further into the 60 m wide openings. The effects were strongest on the shadier aspects of the openings. Overall, openings of 15 m width are not recommended for sites similar to those in the study.
Vegetation competition reduced the growth performance of seedlings that became overtopped early by shrub species-mainly red raspberry, thimbleberry (Rubus parviflorus), and black twinberry (Lonicera involucrata). The presence of hardwood tree species has increased since harvest, but was a minor component (12%) of the overtopping vegetation that caused a reduction in the planted seedling performance. Site preparation (specifically disc trenching) did not affect vegetation response.
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Updated May 11, 2007